The present invention generally relates to radiotelephone communication systems and more specifically to a method and apparatus for networking multiple cellular radiotelephone systems each covering adjacent geographical areas.
Mobile radiotelephone service has been in use for some time and traditionally has been characterized by a central site transmitting by way of high-power transmitters to a limited number of mobile or portable transceivers in a large geographic area. Mobile and portable transmissions, due to their lower power transmitters, were generally received in previous systems by a network of satellite receivers remotely located from the central site for receiving and relaying mobile and portable transmissions to the central site for processing. In previous systems only a limited number of radio channels were available, thus limiting the number of radiotelephone conversations in an entire city to the specific number of available channels.
Modern cellular radiotelephone systems have a comparatively large number of radio channels available which, further, can be effectively multiplied by reuse of the radio channels in a metropolitan area by dividing the radio coverage area into smaller coverage areas (cells) using low power transmitters and coverage restricted receivers. Such cellular systems are further described in U.S. Pat. Nos. 3,906,166 and 4,268,722. The limited coverage area enables the radio channels used in one cell to be reused in another cell geographically separated according to a predetermined plan, such as a four cell pattern shown and described in U.S. Pat. No. 4,128,740. In this four cell pattern, each cell is assigned a subset of the available radio channels and reuse of the radio channels is accomplished by repeating the pattern throughout a metropolitan area.
A cell system typically utilizes a pair of radio frequencies for each radio channel in each cell. Each cell is assigned at least one signalling channel (also called control or access channel) and several voice channels. The signalling channel is selected or dedicated to receive requests for service from mobiles and portables, to page selected mobiles or portables, and to instruct the mobiles or portables to tune to a voice channel where a conversation may take place. This signalling channel is continuously assigned the task of receiving and transmitting data to control the actions of the mobiles and portables. The data message and radio channel specifications for U.S. cellular radiotelephone systems are set forth in Electronic Industries Association (EIA) Interim Standard CIS-3 implemented in accordance with 47 CFR 22 and the Report and Orders pertaining to Federal Communications Commission Docket 79-318. Copies of EIA Interim Standard CIS-3 may be obtained from the Engineering Department of the Electronic Industries Association at 2001 Eye Street, N.W., Washington, D.C. U.S.A. 20006.
Since the cells may be of relatively small size, the likeihood of a mobile or portable travelling out of one cell and into another cell is high. The process of switching the established call from one cell, the source cell, to another, the target cell, is known as handoff. The cellular system determines the need for a handoff by periodically measuring the signal strength of each active mobile or portable. If the measured signal strength is below a predetermined level, the cellular system determines the availability of other channels in neighboring target cells and composes an instruction to the mobile or portable commanding it to tune to an available channel in an acceptable target cell.
As more cellular systems are installed, intersystem handoff between different cellular systems will be required in order to maintain a call as the mobile or portable passes from the coverage area of one system to another. Intersystem handoffs are complicated by the fact that adjacent or colocated cellular systems may be operated by different entities. As a result, each intersystem handoff must be coordinated not only between commonly-owned cellular systems but also between competing cellular systems. Accordingly, there is a need for an improved method and apparatus for networking adjacent cellular systems in order to facilitate intersystem handoff.